This invention relates to a constant-frequency control circuit for a switching voltage regulator of the hysteretic type. In particular, the invention relates to a control circuit for controlling a switching regulator, which circuit comprises a logic circuit driving an output stage, and a hysteresis comparator comparing the output voltage value of the regulator with a reference voltage.
As is well known in this specific field of application, voltage regulators of the switching type are commonly utilized in the power supply to microprocessors.
The current drain to a microprocessor incorporated in a portable computer (notebook), e.g., of the battery-operated variety, is bound to jump up from near zero to a maximum value when the microprocessor is switched from a standby condition to a normal condition of operation.
Due to the parasitic resistance of a filter capacitor, connected to the output of the voltage regulator wherethrough the microprocessor is powered, the output voltage Vo of the regulator shows a drop at the leading edge of said current. In view of this, the regulator is required to respond as quickly as possible in order to regain the voltage drop.
The output stage of a switching regulator comprises a complementary pair of MOS power transistors, e.g., a high-side transistor HS and a low-side transistor LS, which are connected in series with each other. Obtaining a fast response from the switching regulator upon a drop occurring in the output voltage means essentially that the MOS transistor HS must be turned on, and the MOS transistor LS turned off, as quickly as possible.
Control circuits have been proposed in the past wherein the MOS transistor HS is turned on synchronously with a clock signal to impose a predetermined switching frequency. However, circuits to this design are regarded too slow because of a time delay Ts occurring, between a load transient and the turning on of the MOS transistor HS, which can equal the switching period.
Another requirement of a switching regulator is that it should include arrangements to reduce pulse skipping when the load current is too small. In fact, battery life is affected by the power dissipated to the switching operation.
A prior solution described in U.S. Pat. No. 5,825,165 to Micro Linear Corp., for example, proposes that the output voltage Vo present on the filter capacitor be compared with a reference voltage Vref using a hysteresis comparator. A regulator equipped with such a comparator is termed xe2x80x9chystereticxe2x80x9d.
The comparator thresholds are Vref+xe2x88x92Vh/2.
Thus, the comparator output is utilized to drive the MOS power transistors of the regulator output stage.
The comparator switching is produced by ripple on the output voltage Vo originating from a voltage drop through the parasitic resistance of the output capacitor.
This prior solution has adequate speed of response, but the switching frequency is heavily dependent on such parameters as inductance, parasitic resistance (ESR) of the output capacitor, and input and output voltages. In addition, the switching frequency is not constant through a load transient.
According to principles of the invention, a constant-frequency control circuit for controlling a switching voltage regulator is provided. The present control circuit has structural and functional features appropriate to enable the regulator to respond promptly to any drops in the output voltage, as well as appropriate to maintain a low switching frequency while the load current is relatively small.
The circuit employs a regulator which includes a hysteresis comparator, and providing it with a closed-loop control circuit capable of holding the steady-state frequency at a predetermined reference value, irrespective of the parameters affecting the regulator operation.
The control circuit comprises a device for adjusting the hysteresis range of said hysteresis comparator. In one embodiment, it includes a hysteresis frequency sensing and controlling logic portion connected to the output of said hysteresis comparator, said logic portion being arranged to act on said hysteresis range adjusting device.
The features and advantages of the control circuit according to the invention will be apparent from the following description of an embodiment thereof, given by way of example and not of limitation with reference to the accompanying drawings.